The scanning is case sensitive, so if the first letter is capital it doesn't detect it.

[YossefGamerr]

Is there a way to make the scanning not case sensitive?

if the word is written "Vrouw" it isn't detected, because in wikitionary it's written as "vrouw"

[StefanVukovic99]

What version of yomitan are you on? (You can check via the About button in the bottom left of settings)
If you select Dutch from the language dropdown, it should handle capitalization.